The present invention relates to novel copolymers which are copolymers of 4-hydroxystyrene and alkyl substituted 4-hydroxystyrene. This copolymer has been found to be useful in various applications such as binder resins for photoresists.
In a preferred embodiment the invention relates to poly(3,5-dimethyl-4-hydroxystyrene-co-4-hydroxystyrenes) which have a molecular weight in the range of from about 800 to about 100,000 and are suitable as compounds which serve as binder resins in radiation-sensitive mixtures.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,475, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired. Polyvinyl phenols are taught in U.S. Pat. Nos. 3,869,292 and 4,439,516.
U.S. Pat. No. 4,075,237 describes methyl-1,4-dimethyl-2-hydroxystyrene, while U.S. Pat. No. 4,565,846 teaches the use of poly(3,5-dimethyl-4-hydroxystyrene). U.S. Pat. No. 4,221,700 describes a stabilized synthetic polymer composition using poly(alkylated alkenylphenol) including poly(2-methyl paravinyl phenol). U.S. Pat. Nos. 4,600,683 and 4,543,397 describe poly(alpha-methyl vinyl phenol). U.S. Pat. Nos. 4,517,028; 4,460,770 and 4,539,051 describe poly(dimethyl vinyl phenol). The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
The overwhelming majority of commercial lithographic resists perform on the basis of this dissolution inhibition principle. This method uses a resin into which one or more components are added to impede the development of the resin. During exposure or some other processing step the inhibition mechanism is destroyed allowing discrimination in development rates between the exposed and unexposed portions of the resist film.
Novolac resin is the most widely used photoresist resin since it combines the desirable aqueous base development rates with the high reactive ion etch resistance associated with aromatic polymers. However for optical lithography, the use of novolac is limited to radiation at wavelengths longer than 300 nm because of its high absorbance at shorter wavelength. The adverse effect of absorbance results in resist images with severely sloping wall profiles. Phenolic resin use is acceptable for deep-uv lithography only if desirable dissolution properties can be combined with high optical transmission.
Other aromatic phenolic resins have been tried in order to overcome the limitations of novolacs. P-cresolic novolac resins were prepared and it was found that the deep-uv optical properties were improved over regular novolac. However the dissolution rates for resists prepared from these resins were not optimal for lithographic applications.
Nearly complete sensitizer destruction was necessary to obtain reasonable dissolution rates for exposed films. As a result, the sensitivity requirement was too great for practical usage.
Poly(4-vinylphenol) has improved optical properties compared to novolacs but the dissolution inhibition of this resin is disadvantageous. Moreover, the discrimination between exposed and unexposed development rates is quite low with diazonaphthoquinone sensitizers. The rate of dissolution can be altered by incorporation of alkyl substituents on the phenolic pendant groups of poly(4-vinylphenol) without any ill effect on the optical characteristics. The affect of various alkyl substituents showed that alkyl groups do affect the interaction energy and can be used to effect the dissolution properties.
Poly(2,6-dimethyl-4-vinylphenol) has been prepared and evaluated for lithographic application. While the optical characteristics were within the acceptable range, the dissolution kinetics were disadvantageously slow. Reasonable dissolution rates could only be obtained by the use of strong alkali developers which also had undesirable effects on metals used in the relevant processes.
We now find that copolymers comprised of varying ratios of 4-vinylphenol and alkyl substituted 4-vinylphenol are able to combine acceptable optical properties with desirable dissolution kinetics. The alkyl groups can be any of a number of substituents and one or more of these groups can be incorporated at the 2, 3, 5, or 6 positions of 4-vinylphenol. The polymer can be comprised of varying ratios of 4-vinylphenol and one or more alkyl substituted 4-vinylphenol. The ratio of monomer units in the polymer is used as a means of controlling the polymer properties. Increasing the percentage of 4-vinylphenol increases the development rate whereas the alkyl substituted units retard the rate. Furthermore, a variety of molecular weight ranges of these materials can be prepared and blended to produce optimal dissolution properties.
As an example of this approach, the 1:1 copolymer of 4-vinylphenol and 2,6-dimethyl-4-vinylphenol is prepared.
One micron films of these copolymers have an optical density of 0.2 at 254 nm. The improvement over novolac (0.5um.sup.-1) is very significant and makes deep-uv lithography possible. Furthermore the materials show a higher glass transition temperature (160.degree. C.) than novolac (Tg=90.degree.-120.degree. C.) which extends the useful temperature range of these materials without the need for crosslinking.
In most instances, the exposed and developed substrate will be subjected to treatment by a substrate etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern on the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on the substrate produced by the method described above is useful for various applications including as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components. The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion. Resist resolution refers to the capacity of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist. Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Therefore, positive resists are the choice of the manufacture of densely packed integrated circuits. Hence the invention provides novel polymers and photographic compositions employing these polymers. In one important aspect of the invention, it has been found that photoresists which employ the aforesaid polymer as a binder resin show comparable development and dissolution properties to phenol formaldehyde novolak resins but an advantageously lower dissolution rate than polyvinyl phenols which are also known for this purpose.
As hereinbefore mentioned, the polyvinyl phenols have the disadvantage of faster dissolution, which makes image differentiation more difficult to attain. However, they are capable of withstanding much higher processing temperatures, on the order of about 200.degree. C., than the novolaks. The preferred poly(3,5-dimethyl-4-hydroxystyrene-co-4-hydroxy styrene) of this invention demonstrates a dissolution rate comparable to the novolaks and can withstand processing temperatures comparable to the polyvinyl phenols. It is an improvement over poly (4-hydroxy styrene) which typically can only be produced in a low molecular weight, off-color, impure form.
This presents the goal of preparing copolymers for use in a mixture with the other components of a radiation-sensitive mixture and not having the above-described disadvantages of the prior art resists. The nature of the alkyl group can be used to control the dissolution rate of the resin in aqueous base. While the homopolymers of alkyl-4-vinylphenol proved to be too sluggish, copolymers of 4-vinyl phenol and alkyl substituted 4-vinylphenol are observed to have acceptable dissolution rates. Furthermore, the thermomechanical properties of this material are greater than those of novolac and thus higher temperature processes can be carried out without concern over distortion of the image due to flow.